Immunoaffinity chromatography was developed to isolate the cellular phosphoprotein p53 from a mouse neuroblastoma cell. This p53 was stable, not complexed to other protein and had methionine labeled tryptic peptides very similar to the p53 isolated from mouse embryo cells. A method was developed for the quantitation of the p53 mRNA employing a cDNA clone and Northern blot hybridization. A single polyadenylated mRNA species migrating at ca 18S was found in SV40 transformed mouse fibroblasts in the neuroblastoma cells, in embryonal carcinoma cells and also in mouse embryo cells. The level of p53 mRNA was measured in different stages of mouse embryogenesis. Several SV40 transformed cells were found in which the p53 is not in complex with the T antigen. These cells included human placenta cells, human osteosarcoma cells and a set of mouse embryo fibroblast cells. In these latter cells it was shown that the absence of complexing correlates with a low degreee of phosphorylation of the p53.